The present invention relates to methods for making silicone resins. More particularly, the present invention relates to methods for producing silicone resins comprising monofunctional units in the form of R.sub.3 SiO.sub.1/2 units and tetrafunctional units in the form of SiO.sub.4/2 units. The methods of the present invention are characterized by a continuous process for producing silicone resins while maintaining a constant ratio of reactants thus producing less variation in the final product. The present invention is further related to the compositions produced by the methods of this invention.
Silicone resins consisting of triorganosiloxane units (R.sub.3 SiO.sub.1/2) and SiO.sub.2 units are known, commercially available materials and are employed in the formulation of silicone products such as adhesives and antifoams. Such resins are sometimes referred to as MQ resins in view of the presence of the monovalent (M) siloxane units and quadrivalent or tetravalent (Q) SiO.sub.2 units. Resins of this type wherein the organic groups are alkyl and methods for making them have been described in the art. For example, Rochow in U.S. Pat. No. 2,258,218 discloses a method for producing methyl silicone resins by mixing silanes having the formula Si(X).sub.4 wherein X is a halogen such as chloride with a methyl Grignard to make MeSi(X).sub.3 which is then hydrolyzed to produce a methyl resin.
Hyde in U.S. Pat. No. 2,441,320 discloses a method of producing MQ and PhMe.sub.2 Q polymers by slowly adding RMe.sub.2 SiCl(OEt) wherein R is an alkyl group and Et is ethyl, and water to a silane such as tetrachlorosilane or tetraethylorthosilicate (TEOS). Two monosilanes are concurrently hydrolyzed, the hydrolysis products are then dehydrated, and the organosiloxane produced thereby recovered. Daudt et al. in U.S. Pat. No. 2,676,182 teaches a method of producing organic soluble copolymeric siloxanes by reacting silanes or siloxanes with a silica hydrosol. The silica hydrosol is prepared by reacting a water-soluble alkali metal silicate with an acid.
Modic in U.S. Pat. No. 3,017,384 teaches a method of synthesizing MQ resins by adding silanes such as trimethylchlorosilane and ethyl orthosilicate to a solvent such as toluene, this mixture is then hydrolyzed by slowly adding water to it, next the mixture was allowed to separate and the acid layer removed. A silanol endblocked polydimethylsiloxane fluid was then added to the neutralized filtered toluene-resin solution. It is further disclosed that the final resin product could then be cured into a film.
Nelson in U.S. Pat. No. 3,284,406 discloses the synthesis of MVQ (where V is a vinyldimethylsiloxy monovalent group) resins by cohydrolysis of constituent monomeric chloro or alkoxy silanes. The hydrosol may also be prereacted prior to the addition of silane with a vinyl cap before the trimethyl cap is added. Brady in U.S. Pat. No. 3,627,851 teaches a flexible coating of vinyl polydimethylsiloxane gum and MHQ (where H is a dimethylsiloxy monovalent group) resin made by the method of Daudt et al. described hereinabove.
Flannigan in U.S. Pat. No. 3,772,247 teaches the preparation of MHQ resins (precise MQ ratios given) or MDQ (where D is a dimethylsiloxy divalent group) resins or MTHQ (where T is a methylsiloxy trivalent group) resins by a variety of methods which include the method of Daudt et al. hereinabove wherein a sol is prereacted, next adding silanes, and then equilibrating the resulting mixture. Suzuki in U.S. Pat. No. 4,677,161 teaches that hydrogen and methoxy endblocked MQ resins can be prepared by cohydrolysing an alkoxysilane with TEOS in the presence of a strictly controlled amount of water and that the use of an acid catalyst is optional.
Shirahata in U.S. Pat. No. 4,707,531 discloses a method for producing an MQ resin by dripping an alkyl silicate (alkyl orthosilicate or a partial hydrolysis condensate of alkyl orthosilicate) into a mixture of aqueous hydrochloric acid which contains at least 5% hydrogen chloride and a trialkylsilane or a disiloxane or a mixture thereof, at a temperature of from 0.degree. to 90.degree. C. with stirring. Shirahata further teaches that polydispersity and molecular weight can be controlled by the ratios of M and Q added during hydrolysis.
Butler in U.S. Pat. No. 4,774,310 discloses a method for making MQ resins which are Me.sub.2 H and Me.sub.3 capped by reacting in the presence of an acidic catalyst an MQ siloxane resin and a disiloxane. Butler further describes the use of heterogeneous acid catalysts and also discloses post resin synthesis equilibration using strong acid catalysis to add Me.sub.2 H functionality.
Mutoh in U.S. Pat. No. 4,855,381 discloses a method of making MQ resins by reacting an alkyl orthosilicate or a partial hydrolysis product thereof with an oligomeric organopolysiloxane in the presence of a catalyst. The catalyst is disclosed as being an acid such as a sulfonic acid compound or a phosphonitrile compound, an ion exchange resin acid catalyst, or a Lewis acid equilibration catalyst.
Piskoti in U.S. Pat. No. 5,013,808 discloses a method for preparing an alkoxysilane and a silicone resin by combining a cyclic or straight chain siloxane A with a catalyst in a reaction vessel and a silane monomer B having tri or tetra alkoxy functionality, distilling from the vessel an alkoxysilane product of a rearrangement reaction having a boiling point which is lower than the boiling point of the mixture A and B to drive the reaction to completion, and recovering from the vessel a silicone resin having alkoxy functionality and no active silanol functionality.
In contrast, the present invention is directed to a continuous method of producing silicone resins. In addition, the method of the present invention maintains a constant ratio of reactants thus producing less variation in the final product. None of the advantages of the method of the instant invention are disclosed by the references described hereinabove.